The invention relates to a method for maintaining a group call in a mobile radio system, wherein the geographical area covered by the system is divided into radio cells in such a way that adjacent cells overlap, each radio cell comprising a base station provided with an identity, each base station being in radio communication with mobile radio stations located in the radio cell, a group call marked with an identity being established between the mobile stations belonging to a group call group and residing within the areas of the radio cells forming a limited group call area.
The invention is applicable, for instance, in trunking networks, that is, in networks which are private mobile radio networks sharing radio channels to offer several user groups similar functions and services as a private mobile radio network used by a single organization.
The invention is suitable for use in mobile radio systems having either a digital or an analogue radio path. Analogue mobile radio systems are described e.g., in MPT 1327, A Signalling Standard for Trunked Private Land Mobile Radio Systems, January 1988, revised and reprinted November 1991, and MPT 1343, Performance Specification, January 1988, revised and reprinted September 1991, both issued by the Radiocommunications Agency of the British Department of Trade and Industry.
An essential feature of the above-mentioned mobile radio systems, among others, is that they allow and are designed for the implementation of a group call between a plurality of subscribers.
The group call is a conference call during which all participating subscribers are able to both speak and listen to each other in turns. In group calls, the entire group is called by a single call number. An individual radio unit (such as a mobile station) or a subscriber station may belong to several groups programmed in the radio unit. The system contains a database storing information about base stations associated with the number of each group. The group call may cover one, several, or all of the base stations within the area of a mobile exchange, or several mobile exchanges. To establish a group call, each base station belonging to the group allocates a traffic channel and transmits a group call paging message containing the group number and information indicating the allocated traffic channel. If a mobile radio identifies the group number contained in the group call paging message, it switches to the traffic channel indicated by the paging message. In principle, the mobile radio is thus always able to enter the group call if it is located within the predetermined operation area of the group.
In a cellular mobile radio system, the coverage areas (radio cells) of neighbouring base stations often overlap significantly. Mobile radios located within an overlapping area of two adjacent coverage areas are thus able to use either one of the base stations. A situation problematic for the group call is shown in FIG. 1. In the figure, a mobile station MS1 initially located within a coverage area K1 of a base station BS1 belonging to a group call area K1, K2, K3 and involved in the group call moves to a point X within the coverage areas K2, K4 of two neighbouring base stations BS2, BS4, respectively. This is indicated by the arrow T1. The mobile station thus moves, first, to where the coverage area K4 of the base station BS4 overlaps with the coverage area K1 of the base station BS1, then to where the coverage areas K4, K1 and K2 of the base station BS2 all overlap, then to where the coverage area K2 of the base station BS2 belonging to the group call area K1, K2, K3 overlaps with K4, the coverage area of the base station BS4, which does not belong to the group call area K1, K2, K3. In other words, the mobile station MS1 moves to an overlapping area O of the coverage areas K2, K4 of the base stations BS2, BS4, respectively. If the mobile station MI1 now selects the base station BS4 not belonging to the group call area K1, K2, K3, e.g., due to the higher field strength of the transmission of the base station BS4, the group call of the mobile station MS1 is interrupted, and the mobile station MS1 is excluded from the group call even though it actually is located within the group call area K1, K2, K3, as it unfortunately registered to communicate with the "wrong" base station BS4. In FIG. 1, the communication is shown by tie arrow C1.
One way to solve this problem is to use the same radio channels within the same group call area K1, K2, K3 at all base stations BS1, BS2, BS3 in the radio communication between mobile stations M1, M2, M3, M4, M5 involved in the same call and the base stations B1, B2, B3. Such a system is called a quasisfnc or simulcast system. If the mobile station MS1 moves in this kind of system from the coverage area K1 of the base station BS1 to the coverage area K2 of the adjacent base station BS2, e.g., to point X, it is all the time able to communicate with the base stations on the same radio channel. No interruptions occur in the radio connection, and there is no risk that the mobile station would start to communicate with the base station BS4 located outside the coverage area K1, K2, K3, and, as a result, be excluded from the group call.
A drawback of the above-described prior art method and system is that when the mobile station MS1 is located between the base stations BS1 and BS2, it receives a signal from both base stations BS1, BS2, and these signals interfere with each other. To avoid such interference, the transmitters of both of the base stations BS1, BS2 have to be tuned to an accuracy of a few tens of hertz, whereas a conventional tuning accuracy in analogue cellular radio systems is .+-.1 kHz, which means that no high accuracy is required in conventional practice. Of course, tuning to a very high accuracy is difficult to realize.
Another drawback of the prior art solution is that it is poorly suited for modern cellular radio systems which are typically based on the assignment of different channels to adjacent base stations whereas, and the same channels are reassigned at spacings of a few base stations. If a simulcast system is used in such an environment, channel management is difficult to perform in the same way as in a conventional cellular radio system. This is because the simulcast system assigns the same channel to all base stations within a group call area for a specific group call, thus deviating from the conventional principle applied in cellular systems, where the channels vary from one base station to another. The channel management in a simulcast radio system is thus expensive and difficult to implement as well as unreliable in operation.
Furthermore, implementation of the prior art simulcast radio system requires that the interference occurring in transmissions received at the same mobile station over the same channel from different base stations should be eliminated. This is done by compensating for transmission delays of calls forwarded from the system controller of the cellular radio system to the different base stations, i.e., delays occurring over the same voice channel, and by synchronizing such delays so that the same voice channel is transmitted simultaneously from all base stations transmitting a specific group call. Such compensation and synchronization of transmission delays are difficult and require high precision as well as high equipment investments.